Alarm system



D. MITCHELL May 28, 1940.

ALARM SYSTEM Filed Dec. l2, 1936 m m A asc /N VEA/Tof? D. M/ TCH E L L MMM A TTORNEV Patented May 28, 1940 UNITED STATES PATENT OFFICE ALARM SYSTEM Application December 12, 1936, Serial No. 115,641

4 Claims.

This' invention relates to signaling systems and particularly to burglar alarm systems.

The object of the invention is a system capabl of protecting a large area and in which the dangerof an intruder defeating the system is reduced to a minimum. I

In an alarm system, according to the invention, the area to be protected is traversed by a modulated beam of energy, preferably a light beam from which the visible 4rays have been removed by suitable filters. The alarm mechanism is controlled by a relay normally tending to operate the alarm, but held unoperated by opposing windings associated with selective filters connected to a translating device intercepting the beam. At least one of the windings is normally energized by energy derived from the beam and any interruption or substantial variation in the` modulations of the beam permits the relay to operate and sound the alarm.

In one embodiment of the invention,.the beam is modulated at a single frequency and the filter associated withthe winding which holds the relay unoperated is designed to pass only currents of this frequency. A second filter which excludes only currents of this frequency is connected to the other winding of the relay. Upon interruption of the beam the relay operates under the control of its own biasing means and if another source of modulated light is substituted in an attempt to hold the relay while the protected area is invaded, any components of frequencies other than the proper modulating frequency will be passed by the second filter and will aid the biasing means to operate the relay.

In an alternate construction the beam is modulated at two frequencies and at dierent ampli-V tudes, the filters are tuned to the two frequencies, respectively, and the, gains of the associated currents are adjusted so that the opposing windings and the biasing means hold the relay unoperated. An interruption of the beam permits the biasing means to assume control as in the previous case and a substituted beam will be ineffective unless modulated at the same two frequencies and relative amplitudes as the normal beam.

In the drawing:

Fig. 1 shows an alarm system in which the protective beam is modulated at a single frequency;

and

Fig. 2 shows a modified system using two modulating frequencies of predetermined amplitudes.

In Fig. 1, a beam oflight from the source I i: modulated by a light valve II which is driven by an oscillator I2 generating any suitable frequency F1 such, for example, as 500 cycles. The light valve may be of the type commonly used in film recording and the oscillator may be ofthe vacuum tube or "other known' type and is preferably of adjustable frequency, but not subject to appreciable variation for any given adjustment.

The modulated beam passed by the light valve is focussed by a lens I3 on a suitable light filter I4 which removes all visible light rays and passes a beam of infra-red or other invisible energy through the lens I5 to a series of mirrors I6 disposed about the area I'I to be protected, in such a way that no access to that area may be obtained without intercepting the beam. The use of an invisible beam makes it much easier to conceal the systemand in most cases an intruder will sound the alarm before even discovering the location of the protective apparatus. By modulating lthe beam it is made effective over much greater distances than a steady beam so that a larger number of reflecting mirrors may be used and a greater area may be protected by a single system. After traversing the area the beam is intercepted by the lens I8 which focusses it on the photoelectric cell I9 or other suitable translating device responsive to the particular beam used.

The device I9 passes a current varying in accordance with the modulation of the received beam from the battery through the transformer 2i in the input of the amplifier 22. Connected to the output `of the amplier is a filter 23 comprising a circuit tuned to pass only the frequency F1 at which the protective beam is modulated and a filter 2li comprising a network which rejects frequency F1 and passes all other frequencies. These filters are preferably adjustable so that the modulating frequency F1 may be changed from time to time to avoid the possibility of unauthorized persons learning which particular frequency is being used.

The outputs of the iilters are rectified by suitable rectiiiers 25 and 2li which are associated with the windings 2l and 28, respectively, of the relay 29. The relay windings oppose each other and the armature is biased toward the contact 3| by the spring 32, but it is normally held on contact 33 by the energy derived from the protective beam. Under normal conditions no current'iows k in the winding 28 and the currents in winding 2l are adjusted by the potentiometer. 34 to a value suii'icient to overcome the tension of the biasing spring.

A small current normally iiows from the grounded battery through the resistor 36 to ground through contact 33 on the relay 29. Un-

der this condition both sides of the winding of re- 'lay 3l are grounded and the relay remains unop- -sounded immediately upon the breaking of contact 33 this system is very sensitive and will function on very brief interruptions of the protective beam.

With this system it is very difficult to hold up" the alarm by means of a second light source while the intruder is entering the protected area. A steady light source such as a fiashlight will 'obviously be ineffective since a steady current in the primary of transformer iid produces no current in the input of amplifier 22. If the lintruder uses some means for producing amodulated beam it is very unlikely that he will in the first instance choose the correct modulating frequency F1 and currents of any other frequencies will be passed by the filter E4 to the winding 28 of the relay 29 thereby sounding the alarm. By adjusting the potentiometer 4l to give a high effective gain in the circuit supplying the winding 28 the alarm is sounded even when the correct modulating frequency is used if there is any considerable harmonic content in the modulated beam. It will also be understood that the frequency of the oscillator l2 and the constants of the filters may be changed at intervals to make it still more difficult. for unauthorized persons to defeat the system.

The system of Fig. 2 is in many respects sim- 4 ilar to the one described above and similar apparatus has been designated as in the system of Fig. 1. In Fig. 2, however, the light valve is vibrated at two frequencies Fr and F2 simultaneously by the oscillators 5I and 52. By means of the potentiometers 53 and 54 the two component frequencies may be adjusted to any desired relative amplitude.

The output circuit of the light sensitive device I9 includes the transformer 55 which is connected to the opposed windings 51 and 58 of the relay 59 through the amplifier 56, potentiometers y 60 and 61, fltrs 62 and 63 and rectifiers 6l and- 65, respectively. The filter 62 is tuned to pass only frequency F1 and the filter 63 passes only frequency F2. 'I'he potentiometers 60 and 6| are adjusted so that the armature B6 of the relay 59 is normally held midway between the contacts 61 and 68, the effect of the current in winding 58 being sufficient to balance the current in winding 51 and the pull of the biasing spring 69.

Upon4 interruption of the beam the spring 69 closes the relay on contact 61to complete the circuit of relay 10 which in operating locks up and operates the alarm 1l. Similarly, any substituted steady beam or any beam modulated at @dimmed a single frequency will unbalance the relay 59 and operate the alarm and from the foregoing description it will be apparent that no substituted beam will be effective to prevent the relay 59 from closing one or other of its contacts `unless it is modulated at the same two frequencies and at the same relative amplitudes as the normal beam.

While the invention has been described with reference to particular embodiments for purposes of illustration, it will be obvious that various modifications may be made within the scope of the following claims.

What is claimed is:

l. In an alarm system, a source of light projecting a beam, means for modulating the beam at a predetermined frequency, means for filtering out the visible rays of the beam, a receiving circuit including a device responsive to the beam, an alarm, operating means for the alarm, a relay normally tendingto operate to cause said operating means to ,operate the alarm, means in the receiving circuit conductive only to currents of the modulating frequency for disabling the relay, and means responsive to currents of other frequencies in the receiving circuit for operating the relay.

` 2. In an alarm system, a source of light projecting a beam, means for modulating the beam at two predetermined frequencies, means for filtering out the visible rays of the beam, an alarm, a relay normally tending to operate the alarm and having opposing windings, a receiving device responsive to the beam, two output circuits for the device connected to the respective relay windtranslating device at said two frequencies nor-l Amally disabling `the alarm operating means.

4. In an alarm system, means for projecting across an area to be protected, a beam of energy of varying intensity, 4a translating device intercepting the beam and generating currents representing the beam variations, an alarm, a relay, means for biasing the relay so that it normally tends to operate the alarm, and two output circuits for the translating device connected to the relay, the first circuit having frequency selective means for passing only currents of a frequency representing normal variations in the beam and being lconnected'to oppose the relay biasing means and the second circuit having frequency selective means offering high impedance to the currents passed by the first circuit and being connected to assist the relay biasing means.

DOREN MITCHELL.

`ings and being conductive only to currents of the 

